In a typical mobile communications system, the data rate experienced by a user is normally determined by the bandwidth which has been allocated, or scheduled, to the user in combination with the signal quality achieved within the scheduled bandwidth.
Both scheduling and power control may be used to control fairness, or the spreading of experienced bit rate between users served by the mobile communications system. In the uplink (UI) power control may be applied such that when the transmit power is reduced for users located near a base station an increase in performance for users located far from the base station is obtained at the cost of a slight degradation in the overall system throughput.
In a typical mobile communications system which rely on a protocol, such as e.g. High Speed Downlink Packet Access (HSDPA) or Long Term Evolution (LTE), only scheduling in combination with a fixed transmit power is used in the downlink (DL), while power control is used in the UL.
In LIE an ULopen loop power control mechanism regulates the transmit power of mobile stations on the basis of path loss, striving to keep the Power Spectrum Density (PSD) at the base station receiver at a constant level, irrespective of how much bandwidth that has been scheduled to the mobile station. The transmit power, Ftx, for a mobile station on the Physical ULShared Channel (PUSCH) may be expressed with the following formula:Ptx=min{PMAX10 log10(MPUSCH)+PO+α·PL}[dBm]  (1)
where FMAX is the maximum transmit power while MPUSCH is the bandwidth scheduled to the mobile station, where the bandwidth is given as a number M of scheduled Resource Hocks (Ms), where M>=1. FO is an estimation of the target received power per RB for users with PL=0, where PL is the path loss between the mobile station and the base station. α is a parameter used for controlling to what extent the path loss is compensated for. The bandwidth MPUSCH can be compensated for to maintain a constant PSD at the base station receiver.
US20080188260 A1 refers to a method and apparatus for providing UL power control in a communication system, where according to one exemplary embodiment, cell edge performance and spectral efficiency may be optimized by measuring at least one system performance metric at a first Node B. The first Node B communicates the one or more measured metrics to other Node Bs and receives one or more corresponding metrics from the other Node Bs. On the basis of the one or more system performance metrics measured by the first Node B and the other neighboring Node Bs an adaptive power control parameter is determined and used for updating an ULtransmit power level for at least one mobile station served by the first Node B. Consequently US20080188260 A1 relies on input based on metrics which have been measured at nodes other than the first Node B.
Methods for avoiding overload during power control represent a well known technique. U.S. Pat. No. 7,620,004 B2 refers to methods of power overload control in communication systems where the SIRtarget is adopted to avoid overload. According to one exemplary embodiment a method of providing power overload control in a communication system having at least one base station serving one or more users of at least one cell is described. A target Signal-to-Interference-Ratio (SIRtarget) may be controlled for inner loop power control implemented by the base station, so as to maintain the loading of the communication system below an overload condition.
It is a well known fact that scheduling can only control fairness within one single cell. To achieve fairness across a plurality of cells, scheduling only works if there is an equal number of users in each cell. However, in real systems, the number of active users varies both over time and between cells. Under such conditions the users with the lowest bit rate often suffer from a combination of bad radio conditions and a high cell load. FIG. 1 is a schematic illustration of the PSD at a mobile station, where a bandwidth (BW) has been scheduled the mobile station. More specifically, in theory, the bit rate (R) of a mobile station depends on the scheduled BW and the resulting signal quality (SNR), such thatR=BW·log2 (1+SNR)   (2)
FIG. 2a illustrates a typical scenario where eight mobile stations are served by a first base station 200a, while a second, neighboring base station 201a is serving only two mobile stations. As indicated in the two diagrams of FIG. 2a, where each of these diagrams is illustrating the PSD of a respective cell, the mobile stations served by base station 200a are each scheduled resources having a relatively narrow bandwidth, while each of the mobile stations served by base station 201a can be scheduled considerably more bandwidth, thereby enabling the mobile stations of cell 201a to make better use of the available resources and to obtain a better data throughput The bandwidth allocation is normally controlled by a scheduler, while the signal quality is controlled by a power control mechanism. In the described scenario, however, the scheduler will have a limited possibility to improve the fairness between the users. None of the power control schemes mentioned above addresses this problem.